Many aircraft utilize a cabin pressure control system (CPCS) to control the pressure of a cabin, an enclosed space within the fuselage of the aircraft containing crew, passengers, and/or cargo during flight. For a high altitude pressurized airplane, the CPCS must prevent decompression of the fuselage. Decompression of the fuselage above certain limits (hereinafter “catastrophic decompression” or simply “decompression”) is considered a catastrophic event by the Federal Aviation Administration (FAA) and the European Joint Aviation Authorities (JAA). FAA and JAA guidelines state that no single point failure, regardless of the probability, may lead to a catastrophic event.
One type of CPCS utilizes a butterfly-type outflow valve, which contains a butterfly plate (disk) rotating on a drive shaft in the bore of a housing of the valve. Butterfly plate design has progressed sufficiently to ensure that the butterfly plate itself cannot crack or break apart and cause catastrophic decompression. Unfortunately, the same does not hold true for valve plate drive shafts. In many prior art devices this was not a problem as the devices were designed such that drive shaft failure was inherently unable to allow the butterfly plate to separate from the valve housing. However, recent design requirements aimed at decreasing leakage of fully closed valves have resulted in designs where the drive shaft is “slanted” relative to the butterfly plate (i.e. not coplanar with the plate) to allow the plate a full, uninterrupted seal about its circumference. Slanting the drive shaft relative to the plate causes a portion of the drive shaft to be exposed, i.e. not completely retained by the valve. With a portion of the shaft exposed, failure of the shaft can result in the butterfly plate separating from the valve housing with the catastrophic decompression being a possible result. Whether or not heretofore recognized as a problem, the possibility of catastrophic decompression due to drive shaft failure in recent designs warrants development of methods and apparatus directed toward preventing catastrophic decompression even if drive shaft failure occurs in such designs.